The present invention relates to transmission of data on a subscriber loop in a public network such as, for example, a telephone network. More specifically, the present invention provides an improvement of standard single pair digital subscriber line (SDSL) technology.
A wide variety of technologies and transmission standards have been developed for transmission of data via currently existing public network resources. A substantial portion of these resources comprise copper twisted pair transmission lines. This is especially true for the final connections to individual subscribers, i.e., subscriber loops. Without other limitations such as core network filters, such copper lines can achieve practical data rates on the order of tens of megabits per second (Mbps). Of course, substantial attenuation occurs at the higher data rates thereby limiting the length of the subscriber loop which may be serviced at such rates. For example, 24 gauge copper supports reliable transmission of data at the DS1 standard, i.e., 1.544 kbps, also commonly referred to as T1, for up to 12,000 feet. By contrast, the same 24 gauge copper will only support the STS-1 standard, i.e., 51.840 Mbps, for lines of less than 1000 feet.
The term "digital subscriber line" (DSL) refers to a modem or modem pair connected by one or more twisted pairs having a specific data frame format and associated transmission rate. The first digital subscriber line technology, referred to as IDSL, corresponds to what is also known as basic rate ISDN. IDSL technology transmits duplex data at 144 kbps over copper lines using a 2B1Q modulation scheme. The modems multiplex and demultiplex the data stream into two B channels (64 kbps each) and a D channel (16 kbps) as described in ANSI T1.601, the entirety of which is incorporated herein by reference for all purposes.
High data rate digital subscriber lines (HDSL) are related to the earlier IDSL using the same modulation scheme to transmit data at the T1 data rate over two twisted pairs as described in ANSI Committee T1 TR-28, the entirety of which is incorporated herein by reference for all purposes. A single pair digital subscriber line (SDSL) is a single pair version of HDSL, i.e., transmitting data at one-half the T1 data rate, i.e., 768 kbps, over a single twisted pair. For both HDSL and SDSL and as shown in FIG. 1, data are organized into 6 ms frames 102 comprising alternating overhead and payload sections 104 and 106. The four payload sections 106 each include twelve 97-bit payload blocks 108, 96 bits (110) of which are data and one bit (112) of which represents block overhead. This works out to the well known data rate of 768 kbps. Overhead sections 104 and 112 represent an additional 16 kbps for an actual transmission rate of 784 kbps.
FIG. 2 is a simplified block diagram of one portion 200 of a standard SDSL, configured as a HDSL Terminal Unit-Central Office (HTU-C) or public branch exchange (represented by modem 204). The central office typically transmits data via twisted pair line 216 to a subscriber premises (represented by modem 206, FIG. 3). The data to be transmitted enters framing circuitry 208 of modem 204 at the raw data rate of, for example, 768 kbps. Framing circuitry 208 organizes the incoming data stream into the 6 ms frames described above with reference to FIG. 1. In order to perform this task, the framing circuitry 208 utilizes a signal generated from a 768 kHz oscillator 210. The signal provided by oscillator 210 to framing circuitry 208 functions as a data clock, and is used in communicating and synchronizing with incoming raw data on line 201. While the framing circuitry 208 organizes the incoming data stream into the 6 ms frames, it generates frame overhead data, which is multiplexed with the raw data, and inserted into the frame at a rate of 16 kbps. During the time that the frame overhead data is being inserted into the frame, the incoming raw data is queued in a FIFO buffer. When the insertion of frame overhead data is finished, the data from the FIFO buffer is then inserted into the frame, followed by incoming raw data from line 201. The framed data are then sent to bit pump 212 where, using a 784 kHz oscillator 214, they are encoded according to the 2B1Q modulation scheme and transmitted via twisted pair line 216 to the subscriber premises.
As shown in FIG. 2, a conventional HTU-C system, requires two different clock sources: one for the data rate and one for the signaling rate. The solution to this requirement is classically solved via two externally provided clock signals, each signal being generated from a separate oscillator. In FIG. 2, the 768 kHz oscillator 210 is utilized as a data clock by the framing circuitry for generating the data frames and for communicating and synchronizing with the incoming raw data. The 784 kHz oscillator 214 is utilized as a signaling clock by the bit pump for modulating the data frames according to the 2B1Q modulation scheme. However, use of two separate clock sources increases cost, complexity, power consumption, and space utilization of the DSL modem. What is desirable, therefore, is to provide a DSL modem having reduced, cost, complexity, power consumption, and space requirements compared to conventional DSL modems configured as HTU-C devices.